Cathodes and anodes for lithium ion secondary battery are produced by coating metal foil with an electrode slurry formulated with a conductive agent, an active material, and a binder. The electrode slurry in this context refers to a slurry with a conductive agent, an active material, and a binder mixed in a solvent.
As the conductive agent, adjacent carbon powder with excellent electronic conductivity, such as carbon black (hereinafter, referred to as CB), is used. As the binder, styrene-butadiene rubber (hereinafter, referred to as SBR) is used for an aqueous solvent and polyvinylidene fluoride (hereinafter, referred to as PVdF) for an organic solvent such as N-methylpyrrolidone (hereinafter, referred to as NMP).
To improve the charge and discharge characteristics of various batteries, particularly lithium ion secondary batteries, a conductive carbon material of carbon nanofiber (hereinafter, referred to as CNF) and a mixture of CB and CNF are used as an electrode conductive agent. Use or addition of CNF is characterized in that high electric conductivity is obtained with relatively low conductive carbon material content. Such characteristics raise expectations. The CNF here generally has a fibrous shape with an outer diameter from 5 to 100 nm and an aspect ratio, indicating a ratio of a fiber length to the outer diameter, of 10 or more.
Conventional production of CNF includes electrode discharge machining, catalytic vapor deposition, the laser technique, and the like. Among them, catalytic vapor deposition is considered to be most suitable as an industrial production method. In the catalytic vapor deposition, transition metal particles are used as a catalyst to make contact a raw material gas as a carbon source, such as acetylene and benzene, thereby growing CNF from catalyst particles generally at high temperatures of 900° C. or more. In particular, methods of producing CNF from a gas as a raw material containing carbon monoxide as a main component by using a transition metal component, such as cobalt, as a catalyst gain attention as a method of producing high purity and high quality CNF at relatively low temperatures (PTLs 1 to 5).
To use CNF as a conductivity imparting agent for lithium ion secondary battery, dispersibility of CNF is important. For a better state of CNF dispersion in the electrode, there is a method of inhibiting aggregation of CNF by dry mixing an active material and carbon fiber and then mixing the dry mixture, a binder, and a solvent (PTL 6).
For a better state of CNF dispersion in the electrode, there is another method of uniform dispersion in a solvent by surface oxidation process to CNF using concentrated nitric acid and concentrated sulfuric acid (PTL 7).